I have tried dowsing once or twice, without any luck whatsoever. I
tried copper tubes and walked at right angles over a buried watermain,
time and again. No twisting, no pulling, nothing.

You ascribe a "positive feedback" (my words) to demonic activity. Let's
step back a bit. Would Satan pick only on people who are looking for
water? Sure, water is a precious commodity, something we can't do
without but it is also an economic resource. Are there techniques
similar to dowsing that can be used to locate ore bodies? Seems to me
that would beat looking for water hands down and might really get Satan
something to aim for.

So, before we set out on an experiment (in which I'd love to
participate!), maybe we should first find answers to the following
questions (among other)

* are dowsing techniques only applicable to water
* must the water be running or can it be stagnant water
* does the TDS (total dissolved solids) concentration in the water
have any effect (saline water compared to potable water)
* does it work underground, e.g. in mines
* is dowsing an acquired skill
* does it work better in humid conditions
* does having a naturally dry skin make it easier or more
difficult to sense any "force"

and, of course, most importantly,

* how many false positive and false negative readings are
obtained?

False negative (where the dowser erroneously finds no water) can of
course only be noted in controlled tests, otherwise you'd have to have a
backhoe follow the dowser. Perhaps dowsing is something like fortune
telling where one only keeps track of the successes, not the failures!